Damper control in space heating and cooling

ABSTRACT

An amount of time that air has been delivered from an air handler to a space is tracked, and based on the tracked amount of time, at least one turn-on time or one turn-off time of the delivery of air from the air handler to the space is controlled. Other features relate, among other things, to controlling a replacement air vent, and to user interfaces.

BACKGROUND

This description relates to damper control in space heating and cooling.

During the 1990s, the United States Department of Energy sponsoredresearch on how to save energy in heating and cooling houses and otherbuildings. As shown in FIG. 1, one recommendation that has begun to bewidely adopted is to super-insulate buildings, seal them tightly againstair infiltration, and use a vent 10 from the outside world 12 to let infresh air. The fresh air is needed to clear odors and humidity from thetightly sealed spaces 14 that are occupied within the buildings. Theenergy savings produced by such a system are so large that it isexpected that, in the future, most new buildings will be super-insulatedand tightly sealed.

As is typical of forced air heating or cooling systems, the heater orcooler 16, 18 (and a central fan 20) is turned on and off in response toa thermostat and controller 22 based on a comparison of a set pointtemperature and a current air temperature measured at a temperaturesensor 24. The central fan 20 forces air from the heater or coolerthrough ducts 26 into the occupied spaces 14. Stale air is withdrawnfrom the spaces through return ducts 27 and returned to the intake sideof the air handler. While the heater or cooler is running, the stalereturned air is supplemented with fresh air that is drawn into thebuilding through the vent 10. A damper 28 inside vent 10 is set in afixed position to permit no more than a suitable amount of fresh air tobe drawn in while the heater or cooler is running.

Even during intervals when the heater or cooler is not running, freshair continues to be needed, and for this purpose, the central fan may berun from time to time during those intervals.

Heating and cooling systems are generally sized so that they run almostfull-time during the coldest or warmest months. When a system that drawsin fresh air from the outside world runs all the time, more air is drawnin than is needed for air exchange purposes, and energy is wasted inheating or cooling it. By motorizing the damper 28, it is possible toopen and close the damper in cycles to reduce the amount of fresh airdrawn into the building. In some systems, a user can specify theproportion of time that the damper is opened to permit fresh air to bedrawn in. A replaceable filter 29 is included in the vent to filter theincoming air.

The cooler and/or heater are part of what is often called an air handler32, which may also include a humidifier and/or a dehumidifier 34, and avariety of other equipment. A variety of configurations are used for airhandlers, the equipment that is in them, and the equipment to which theyare connected.

The air in the air handler can be heated and/or cooled in a variety ofways. A typical cooler includes the heat exchanger 18, a compressor 36located outside the building, a delivery conduit 38 with a pump 40 toforce coolant from the compressor to the exchanger and a return conduit42 to carry used coolant back to the compressor. The pump is controlledby the controller 22.

SUMMARY

In general, in one aspect, the invention features tracking an amount oftime that air has been delivered from an air handler to a space, andbased on the tracked amount of time, controlling at least one turn-ontime or one turn-off time of the delivery of air from the air handler tothe space.

Implementations of the invention may include one or more of thefollowing features. The tracking of the amount of time that air has beendelivered includes tracking the on time of a circulating fan thatdelivers the air. The controlling of the turn-on time or turn-off timeis also based on an intended amount of time that air has been deliveredfrom an air handler to a space. The intended amount of time includes aminimum amount of time. The intended amount of time is specified by auser. A vent that delivers replacement air to the air handler is alsocontrolled to open or close by a selectable amount to achieve aparticular flow rate of replacement air. The particular flow rate isderived from at least one user specified value. The user specified valueincludes an intended average flow rate.

In general, in another aspect, the invention features, based oninformation about an intended flow rate of replacement air to an airhandler and an intended amount of time that air is to be delivered fromthe air handler to the space, performing a calculation to determine aflow rate of replacement air to an air handler to be achieved duringperiods when a vent that controls the delivery of replacement air isopen.

Implementations of the invention may include one or more of thefollowing features. The information about an intended flow rate ofreplacement air includes a value indicating an average volume of air pertime period. The value is specified by a user. The information about anintended amount of time is specified by a user. The calculation includesdividing the intended flow rate by the amount of time. The calculationis also based on information about a duration of a duty cycle of a fanthat delivers the replacement air to the space.

In general, in another aspect, the invention features enabling a user ofa controller associated with an air handler to enter a value of anamount of replacement air to be delivered to a space. In someimplementations, the amount of the replacement air is expressed as anaverage volume per time.

In general, in another aspect, the invention features enabling a user ofa controller associated with an air handler to enter a value of aminimum amount of time that a fan of the air handler is to run. In someimplementations, the user is enabled to enter a value of a duty cycle ofthe fan.

In general, in another aspect, the invention features enabling a user ofa controller associated with an air handler to enter a value of anamount of replacement air to be delivered to a space, a value of aminimum amount of time that a fan of the air handler is to run, and avalue of a duty cycle of the fan.

In general, in another aspect, the invention features enabling a user ofa controller associated with an air handler to enter an indication of atemporary change in an amount of replacement air to be delivered from anair handler to a space.

Implementations of the invention may include one or more of thefollowing features. The user is enabled to enter an indication of theduration of the temporary change. The temporary change includes anincrease associated with an increase in anticipated occupancy of thespace. The temporary change includes a decrease associated with adecrease in anticipated occupancy of the space.

In general, in another aspect, the invention features sensing flow ofreplacement air through a vent to an air handler, opening and closingthe vent to regulate delivery of replacement air to the air handler toachieve an intended rate of flow, and based on the sensed flow,determining a clogging state of the filter.

Implementations of the invention may include one or more of thefollowing features. The clogging state includes the filter beingclogged. The clogging state includes the filter being new. Flow ofreplacement air is sensed by sensing a change in the flow over time.Determining of the clogging state includes comparing the sensed flow toa threshold value of flow. The threshold value of flow is setempirically. The threshold value of flow is set based on the history ofthe sensed flow. The sensing of flow of replacement air is done bymeasuring a rate of rotation of a fan drive by the flow, or by measuringpressure of the air, or by measuring a velocity of the air. The ventincludes a replacement air vent or a return air duct.

In general, in another aspect, the invention features issuing a ventsignal to open a vent that regulates delivery of replacement air to anair handler, and in connection with the issuing of the vent signal,issuing a fan signal to turn on a circulating fan of the air handler. Insome implementations of the invention the fan signal turns on the fanindependently of a thermostat that controls the air handler; in someimplementations the fan is turned on using a relay.

In general, in another aspect, the invention features an apparatusincluding a conduit containing a sensor to measure flow of air in theconduit, an air flow regulator to control flow of the air in theconduit, a coupling to couple the conduit to a fresh air intake of theair handler, and a controller to control the air flow regulator based onthe measured air flow.

Implementations of the invention may include one or more of thefollowing features. The sensor is in the conduit or not in the conduit.The air flow regulator includes a damper. The sensor includes a fan. Theconduit includes a fresh air vent. The conduit includes a return airduct.

In general, in another aspect, the invention features an apparatusincluding, as an assembly, a controllable damper to regulate flow of airthrough an air flow passage between an exterior of a building and an airhandler, a sensor to sense air flow through the passage, and terminalsto connect the sensor and the controllable damper in a control circuit.

Implementations of the invention may include one or more of thefollowing features. The sensor includes a contactless sensor. The sensorincludes magnetic elements. The sensor includes a fan in the passage torotate in response to air flowing through the passage. The rotation ofthe fan is sensed as an indicator of the volume per unit time of airflowing through the passage. The fan includes blades, at least one ofthe blades bears a magnet, and the sensor senses motion of the magnet.The damper is controllable to be open or closed. The damper is alsocontrollable to be open to a selected position among at least twodifferent open positions. The damper is controllable to be open to aselected position with a range of positions. The damper includes arotating flap driven by an electric motor. The fan is free-wheeling. Thecontrol circuit controls opening and closing of the damper in responseto sensed air flow through the passage. A filter filters air flowingthrough the passage. A housing defines at least a portion of the airflow passage and includes ends to mount the housing between an exteriorof a building and an air handler. The housing also supports the damper,the sensor, and the terminals. The housing also supports a filter. Thefilter is mounted on a second structure that mates with the housing. Thehousing also supports at least a portion of the control circuit. Thecontroller includes a circuit that receives signals from the sensor andsends signals to the controllable damper. The control circuit includes aterminal to connect to a thermostat. The control circuit includes aninput to receive an indication from a user of how much air is to bepermitted to flow through the passage. The control circuit includeslogic to control the damper to allow a predetermined volume per unittime to flow in the passage.

In general, in another aspect, the invention features an apparatusincluding a controllable damper to regulate flow of air through an airflow passage between an exterior of a building and an air handler, asensor to sense air flow through the passage, a control circuit toreceive signals from the sensor and to send signals to cause thecontrollable damper to allow a predetermined volume per unit time toflow in the passage, and a housing that supports the damper and thesensor and includes features to mount the housing between an exterior ofa building and an air handler.

In other aspects, the invention features media bearing instructions tocause a machine to perform such techniques and apparatus to perform suchtechniques.

Other advantages and features will become apparent from the followingdescription and from the claims.

DESCRIPTION

FIG. 1 is a schematic diagram of a space heating and cooling system.

FIG. 2 is a three-dimensional view of portions of a heating and coolingsystem.

FIGS. 3 and 9 are a sectional side view and a top view of an assembly.

FIGS. 4 and 5 are perspective views of parts of a damper.

FIGS. 6 and 8 are perspective views of parts of an airflow sensor.

FIG. 7 is a perspective view of a flange/filter housing.

FIG. 10 is a schematic diagram of a control system.

FIGS. 11A, 11B, and 11C are views of a controller.

FIGS. 12 through 15 are time lines.

FIG. 16 is a flowchart.

As shown in FIG. 2, an airflow sensing unit 52 can be placed in the flowpath of outside air 13 (or other source of replacement air) that ispassing from the outside environment 12 to an intake port 54 of the airhandler 32 from an outside air vent 90. (We use the phrase air handlerin a very broad sense to include any kind of equipment that processesair for the purpose of providing, for example, heating, cooling, orventilation in a space.) The air flow sensing unit 52 includes an airflow sensor (hidden in FIG. 2) that produces a stream of signals fromwhich the volume of air that passes along the air path per unit of time(e.g., 20 cubic feet per minute, CFM) may be derived.

The derivation of the CFM can be done, in one example, by a processor ina local electronic circuit 56 (which we sometimes call an airflowcontroller) that is mounted on the sensing unit 52 or, in anotherexample, can be sent by a cable 58 to a thermostat and controller 60(which we sometimes call simply a controller or a main controller)mounted on a wall 62 of a space of a building.

The main controller 60 contains a thermostat circuit that compares dataindicative of the temperature in the space with a desired set pointtemperature. In some implementations, the controller itself may notcontain a temperature sensor but may be connected as a controller to anexisting thermostat and in that role monitors the existing thermostat.The controller 60 sends control signals on a cable 66 to a set ofdrivers 68 on the air handler to control heating and cooling to drivethe temperature in the space to reach the set point and to controlcentral fan operation during heating and cooling and at other times. Thecontroller 60 may also receive data on a cable 70 from an outside sensor72 that senses one or both of the relative humidity and temperature ofthe outside air and may use the data as part of an algorithm thatdetermines when to call for heating or cooling.

(For example, if the controller determines that the outside temperatureis cooler than the inside temperature at a time when cooling is beingrequested, the controller could open the damper fully and turn on thecentral fan for a period to attempt to cool the space with outside airwithout using the cooling feature of the air handler. The conversedetermination could be made for heating when the outside temperature iswarmer than the inside temperature.

If the outside relative humidity is high during a call for cooling, thecontroller could allow the space to be cooled a small amount lower thanthe set point to allow long cooling runs to dry out the inside air.Short cycling the air handler for cooling tends not to remove much waterfrom the air, which can occur if a system is over-sized. In another use,if the outside air temperature is close to the inside air temperature,which could result in relatively little fresh air being provided to thespace, the damper may be open fully or for a longer period to increasethe fresh air delivered.

These control features could also be based on signals from an insiderelative humidity sensor.

In another application, when the weather is cold and dry outside, andthe inside relative humidity is elevated, the controller may open thedamper more fully or for a longer period to reduce the inside relativehumidity.

The main controller 60 also is configured to send damper control signalsto control a motor 78 that is mounted on a damper 50 and can drive thedamper to any position between full closed and full open (the full openposition may be, e.g., 90 degrees from its closed position). The dampercontrol signals may be sent on cable 58 through the airflow controller56 to the motor driver. The controller can open and close the damper forany number, frequency, and length of time periods and by any amountswithin the operating range of the damper. The main controller uses analgorithm and circuitry (discussed later) to determine the time periodsand the degree of opening that will be applied for each time period.

The airflow controller drives the damper to the desired position in thefollowing way: The damper motor may be a 1 rpm motor, for example, sothat the passage of time can be used to determine position. For example,running the motor for 15 seconds puts the damper full open at 90degrees. The motor can be indefinitely stalled without damage, so eachtime the damper is to be closed fully it is run longer than necessaryand stalls in the full closed position, which effectively resets it to aknown position. Because the motor is run on alternating current, whichis closely regulated by the power company, and because the clock speedof the microprocessor is relatively accurate, position can be determinedaccurately based on time.

The damper 50 and the air flow sensing unit 52 have cylindrical outerwalls and are arranged in line together with a flange 82 to form a ventinsert 84. The vent insert can be installed in line with and between astandard vent pipe 86 and the rectangular intake port of the airhandler. The other end of vent pipe 86 passes through a wall 88 of thebuilding and connects to the outside vent cover 90.

As shown in FIGS. 3, 4, and 5, the damper 50 includes a moldedcylindrical body 94 and a molded flat round vane 95. Approximatelyhalfway along the inner wall of the body 94 is a circular rim 96 thatprojects into the space within the cylindrical body to define a closedposition at which the damper is stopped as it is rotated to the closedposition.

On the outer wall of the body 94, a flat surface 98 is defined tosupport an electric stepper motor and gear assembly 100 used to drivethe damper to selected positions based on signals sent from thecontroller.

At two diametrically opposite positions around the rim 96 are two holes90, 92. The vane 95 (which is not shown in FIGS. 3 and 4) has twoslightly offset (along an axis normal to the vane) semicircular plates97, 99, joined at a central tube 91. The damper is held in place in thebody 94 by two pins 93, 97 (FIG. 3), one that projects from hole 90 intoone end of the central tube. One end of the other pin is connected to ashaft of the motor and gear assembly 100. The other end of that pinprojects into the other end of the central tube 91 and is keyed intothat hole so that rotation of the motor causes rotation of the damper.

The circular end 102 of the body of the damper 50 that connects to thesensor unit has projecting fingers 106, 108 that mate with and lock intocorresponding holes 109, 111 (FIG. 6) in a body of the sensor unit. Theother end 103 of the body of the damper 50, which connects to the flange82, has two holes 110, 112 to receive projecting fingers similar to thefingers 106, 108.

Referring to FIG. 7, the flange 82 has a round end 120 having an insidediameter that is slightly larger than the outside diameter of the end ofthe damper with which it mates. Two fingers 122, 123 project into thespace defined by the round end 120 and mate with the holes 110, 112 ofthe damper. All of the fingers 106, 108, 122, 123 have tapered leadingedges to permit then to be easily forced into the mating holes and haveblunt trailing edges to make them hard to remove from the mating holesexcept by inserting a tool through the holes and against the fingers toforce them out of the holes.

The flange 82 includes a square cross-section tapered wall 126 thattapers from the round end 120 to a square cross-section to the oppositesquare end 128 of the flange. The square end is defined by a rail 130that is formed along three sides of the square end. The fourth side 132has no rail.

The rail 130 includes a mounting lip 134, 135 having a row of screwholes for use in mounting the flange to the sheet metal wall of the airhandler. The three sides of the rail define a square pocket at thesquare end of the flange that is larger than the inlet port of the airhandler and is deep enough to receive an air filter (not shown), e.g., astandard square air filter or a custom one.

As shown in FIG. 6, the airflow sensing unit 52 has a molded cylindricalbody 140. One end 142 of the body has a tapered section 144 to enablethe unit to be inserted and held within the inner diameter of the ventpipe 86. The other end 146 of the unit has an enlarged cylindricalsection 148. The inner diameter of the section 148 is large enough toreceive the outer diameter of the end of the damper.

The outer wall of the body 140 supports a box 150. The electroniccircuit 56 (not shown in FIG. 6), which we also call an airflowcontroller, is held in the box. Inside the body 140, four wings 156(arranged at 90-degree intervals) extend from the inner wall of the bodyto a central axis 158. At the central axis, a ring 160 is supported onthe wings. A hole 162 in the ring is sized to receive a pin that is usedto mount a fan.

As shown in FIG. 8, the fan 164 that is mounted on the body 140 has fouridentical fan blades 166 evenly spaced around a hub 168 that has amounting hole 170 and a central axis 172. The fan blades are mounted atan angle to the axis. The hub is mounted on the ring 160 (FIG. 6) usinga pin (not shown) that permits the fan to rotate freely about the axis158, 172. A magnet 173 is mounted near the outer end of each of the fanblades.

As shown in FIGS. 3 and 9, when assembled, one end 103 of the damper 94is inserted into the round end 120 of the flange until the two fingerson the flange latch into the two holes in the damper. The other end 102of the damper is inserted into the larger end 148 of the sensor unit 146until the fingers on the damper snap into the corresponding holes in thesensor unit. The resulting assembly 180 is then installed in thebuilding by screwing the flange to the air handler and inserting thefree end of the sensor unit into the vent pipe. The motor 100 of thedamper is connected to a source of power and the signal lines among theairflow controller and the damper are connected to the main controller.A filter is inserted into the pocket at the interface between the airhandler and the flange.

Once the assembly 180 has been installed, when the damper is open andair is drawn into the air handler from the outside, the air movesthrough the sensor causing the fan to rotate. The fan rotates morerapidly with higher velocity of air motion. The rotation of the fan isindicative of the air flow volume per unit time. As the fan rotates, theairflow controller detects when each of the magnets on the blades passesthe location of a magnetic detector that is part of the airflowcontroller. The airflow controller then determines the RPM (which may bethe instantaneous RPM in some examples, or an averaged RPM in otherexamples). Based on the RPM signals, the main controller converts theRPM signals to a flow rate in CFM, for example, by using a storedlook-up table that associates flow rates with rotation rates asdetermined empirically.

The airflow controller circuitry 202 and the main controller circuitry204 and their interconnections are shown in FIG. 10.

The main controller includes a microprocessor 204, a display 206 that iscontrolled by the microprocessor, and a keyboard 208 that enables a userto manage the operation of the main controller. In one implementation,the keypad provides eight keys (membrane switch keys 1 through 6, andup, down, and mode buttons), and the display has the configuration shownin the figure. The microprocessor includes control outputs 209 for thefan driver 210, the heat driver 212, a second heat driver 214, and acooling driver 216. The outputs are carried on a cable 66 to the airhandler where the drivers are located.

The main controller includes a thermistor 218 to detect the temperaturewithin the space being heated or cooled. The main controller may alsoinclude a relative humidity sensor 220. Optionally, the microprocessorcan also receive signals from an outside temperature sensor and anoutside relative humidity sensor 72 that are mounted in a positionexposed to the outside world. Data to be sent back and forth between themain controller and the airflow controller on the cable 58 is handled bya network interface 222 at the main controller end of the cable and acorresponding network interface 224 on the airflow controller end of thecable.

The airflow controller 202 includes a microprocessor 230, which receivesdirectives about the timing and degree of opening of the damper from themain controller. The primary output control signals from themicroprocessor are clockwise and counterclockwise signals 232, 234 thatare delivered to the motor driver 236. In one example, thecounterclockwise signals are controlled to cause the damper to movetoward the fully open position. The clockwise signals are controlled tocause the damper to return toward the fully closed position. Any degreeof opening between fully open and fully closed can be achieved. Theairflow controller turns on the central fan whenever the damper isopened. In examples that include a thermostat in the central controller,the controller would cause the central fan to be turned on using asignal 233 produced by the airflow controller. In examples in which thecentral controller does not include a thermostat, a relay 225 is used toturn on the fan independently of the thermostat.

The fan sensor 240 may be a Hall effect device that detects the passageof each blade of the fan and delivers a corresponding signal to themicroprocessor. The microprocessor converts the signals to an RPM value,which is then passed back to the main controller through the networkinterfaces.

A pushbutton 242 may be used to test the airflow controller, and atri-color LED 244 is used to indicate the state of the airflowcontroller. Optionally, the airflow controller can receive signals fromincoming air temperature and humidity sensors 248, 246, process thesignals to produce raw data, and pass the raw data back to the maincontroller.

The airflow controller operates as a slave to the main controller andreceives and responds to commands from the main controller.

When the main controller commands the slave to open the damper toposition x, the airflow controller causes the damper to open to therequested position, x. When the main controller commands the slave toreport its status, the airflow controller reports the position of thedamper, including the status indicated by its LEDs 244, the state of thepush button 242, and any error codes. When the main controller commandsthe slave to report the fan RPM, the airflow control sends back thevalue of the fan RPM. When the main controller commands the slave tochange the LED's state, the airflow controller replies with anacknowledgement.

FIGS. 11A, 11B, and 11C show a front view with cover closed, aperspective view, and a front view with cover open of the externalhousing of the main controller. In addition to controlling the fan onperiods and the damper open periods, the controller serves as aconventional programmable thermostat. For this purpose it provides keysto program a weekday set point schedule and a weekend set pointschedule, and keys to set the day and time. A fifth key controls the setpoint and a hold key sets the hold function. The two buttons that haveup and down arrows are used to increase or decrease a value and thesquare button serves a similar role to an enter button on a keyboard.

The mode and up and down buttons are used to set Af, Fp, and Fm values(described later). The controller includes a main housing and a basethat is attached to the wall. The main housing snaps onto the base. Byholding the up button in while snapping the housing to the base, themicroprocessor is alerted to enter setup mode. Once in setup mode thedisplay indicates the value that is being set. Pressing the mode buttoncycles through the three variables that are to be set. When a givenvariable is in set mode, the up and down arrows control the value of thesetting. Other arrangements could be used to invoke the setup mode, forexample, pressing a combination of the membrane switches at one time. Insome implementations, a separate device may be provided to read out datafrom the controller and the device may also be able to lock and unlockthe settings or to re-program the settings and then lock the settings sothat the user is precluded from changing them.

The hold button controls both the hold options and the high occupancyoptions. The hold options could include setting a number of days forholding, or setting to hold indefinitely. The high occupancy optionwould hold the setting for a specified number of hours.

To operate the system, the user may use the keypad and the display ofthe controller to enter several values to be used by the controlalgorithm. One value is an average desired fresh air flow rate into thespace being heated or cooled, called Af and expressed in cubic feet perminute. The user can determine what this value should be by using simplerecommendations of another party or by doing a calculation on a websitebased on the characteristics of the house, and its occupancy. ASHRAE,for example, specifies 15 CFM per person. Or 15 CFM per bedroom+one. Forexample, the user may set the value of Af to 30 CFM indicating a desireto have an average 30 CFM of fresh air delivered to the space. A secondvalue is the controller duty cycle called Fp and expressed in minutes,which represents the durations of the successive periods over which thealgorithm will be applied. A third value is a fan minimum run time,called Fm and expressed in minutes, which represents the minimum numberof minutes that the fan should run during each controller duty cycle.

The controller uses the entered values to calculate a required flowrate, called Ar and expressed in cubic feet per minute, which will applyduring the periods when the fan is running and the damper is open. Ar iscalculated as (Fp/Fm)Af=Ar. For example, if Af=30, Fp=10, and Fm=30,then Ar=90 CFM which is the flow that must be achieved during theperiods when the damper is open.

The user can use the controller keypad to override the normal operationof the algorithm by specifying a hold mode or a high occupancy mode.

The hold mode could be applied, for example, during a vacation periodwhen the space will not be occupied. When the user presses the holdbutton, the controller prompts the user to enter a number of days tohold. The controller then holds the temperature constant at the thencurrent set point and disables setback scheduling for the specifiednumber of days or indefinitely (depending on the setting option that isused. The fresh air flow rate Af is reduced to a pre-set minimum flowrate, for example, 90 CFM. The fan minimum run time Fm is reduced to apre-set time, for example, 10 minutes.

Another variant of the hold mode could be used in situations in whichoutside ventilation is being obtained, say, from an opened window in acontext in which the thermostat is not calling for either heating orcooling. In such a circumstance, when the user enters the hold mode, hecould be given an option to completely disable fan operation and freshair input, for example, until further input from the user.

The high occupancy mode may be used, for example, when a larger thannormal number of people will occupy the space, requiring a higher thannormal fresh air flow rate. When the user presses the high-occupancybutton, the controller prompts for a number of hours to maintain thehigh occupancy mode. During the period when the mode is maintained, thetemperature is held at the current set point, and setback scheduling maybe disabled. The fresh air flow rate Af is increased to a pre-setmaximum flow rate, for example 90 CFM. The fan minimum run time, Fr, isincreased to a pre-set run time, for example, 10 minutes. During highoccupancy mode, if the set point temperature cannot be maintained, thenthe fresh air flow rate Af will be decreased until the set pointtemperature is reached. Reducing the fresh air flow rate in this waywill enable the heater or cooler to adjust the temperature to the setpoint.

As shown in FIG. 12, in some control systems a user can indicate thepercentage of time (for example, 33%) that he would like the central fanof the air handler to run—whether or not the thermostat is calling forheating or cooling—in order to keep air circulating in the space. Suchsystems track off time as a control technique. Note that the fan isalways on when the thermostat is calling for heating or cooling. Duringperiods when the thermostat is not calling for heating or cooling, thesystem monitors the amount of off time. If the amount of off timeexceeds the desired percentage, then the fan is turned on.

For example, as shown in the figure, the user may specify that thecentral fan should run 33% of each 30-minute period. Suppose that thethermostat makes no call for heating or cooling at any time during the30-minute period. Time line 402, in the upper half of the figure, showsthe on and off periods of the fan during. For the first 30 minutes, thethermostat is not calling for heating or cooling and the central fan ison 404 for the first 10 minutes, then off 406 for 20 minutes in order tomeet the desired percentage of on time. The same pattern is repeated inthe second 30 minutes. In this example, the desired proportion of fan ontime, 33%, is accurately achieved.

By contrast, in the time line 408, shown in the bottom half of FIG. 12,the desired proportion of fan on time is not met. In this example, thethermostat calls for cooling for 4 minutes 410, followed by an interval412 of 16 minutes of no cooling, and then the pattern repeats. Duringthe first 4 minute cooling period, the fan runs. When the cooling ends,the fan is turned off. If no cooling were then required for more than 20minutes, the fan would be turned on by the algorithm, which watches theamount of off time to assure that the fan is never off for a periodlonger than 20 minutes. However, in the example, a new cooling period istriggered after only 16 minutes causing the fan to go on, so thealgorithm never determines that the fan has been off longer than 20minutes. The same sequence then repeats. As a result, the fan is only onfor 12 minutes an hour, instead of the desired 20 minutes per hour, anerror of 40% that results in the air in the space being less fresh thandesired.

Referring to FIG. 13, in a different approach, it is the on time of thefan that is tracked and the algorithm assures that a minimum desired ontime per controller cycle is met. For example, the user may select a fanminimum on time of 10 minutes in each 30-minute period, the same targetas in the example of FIG. 12. Suppose that, as in the lower half of FIG.12, the thermostat calls for cooling for 4 minutes at the beginning ofevery successive 20-minute period. In the time-line 420, the fan runsduring the initial 4-minute cooling period 422. At the end of thatperiod, when the fan is turned off, the controller (which is trackingthe on time to see if it meets the desired value) determines that, tosatisfy the desired 10 minutes of fan on time for the first 30 minuteswill require that the fan be operated another 6 minutes no later than atthe last portion of the 30-minute period. At the end of the second4-minute period 424, the controller determines that 8 minutes of theneeded 10 minutes of fan on time have occurred, with two minutesremaining. At the end of an additional 4 minutes of off time 426, only 2minutes remain in the half-hour period, so the controller turns on thefan for a 2-minute period 428 to meet the goal. Next the remaining 10minutes of the 16-minute off period 430 occurs, and the fan remains offduring that period. After the next four-minute off period 432, thecontroller determines that 6 more minutes of fan on time are required inthat half hour. So the controller allows the fan to remain off foranother 10-minute period 434 and then turns it on for the final 6-minuteperiod 436 of the second half-hour. The fan on time then exactly matchesthe desired on time of 20 minutes for the hour.

If, near the end of the system cycle (30 minutes in the above example),the time remaining for the fan to be run is small, say less than 3minutes, the algorithm could decide not to run the fan, or to defer theneeded time to the next cycle. This may reduce complaints by users thatwould otherwise be generated when they hear the fan run for shortperiods of time.

Thus the controller is able to achieve the desired fan on time with noexcess (which wastes power and may take in too much air) and noshortfall (which may leave the air in the space stale).

FIGS. 12 and 13 are focused on the timing of fan on and off periods. Wenow consider how the damper may be controlled to assure that a desiredamount of fresh air is provided to the space. FIG. 14 illustrates thatsome known systems for controlling the open or closed state of thedamper (vent) do not accurately meet the desired proportion of opentime. As shown in the example, in such systems the user can specify theproportion of time that the vent is open, say, 33%, which corresponds to10 minutes open and 20 minutes closed per half hour.

Suppose that, in the example, the thermostat is calling for heat for 10minutes at the beginning of each successive 15-minute period. In theknown system, the vent is open when and only when the fan is operating.Because the operation of the fan to serve the heating need is more thanenough to meet the desired 10 minute per half hour vent open time, thetime line 450 represents the periods when heat is and is not beingcalled for, and implicitly when the fan is running and not running andthe damper is open and not open. In the example, the total fan on timeand hence the total damper open time is 40 minutes during the hour, or66% of the time, which is an error of 100% in the desired proportion ofdamper open time. Because the damper is open more time than is needed,energy will be wasted.

In a different control approach, illustrated in FIG. 15, the userspecifically sets the fresh air rate Af at, say, 30 CFM, the minimum fanrun time Fm at 10 minutes, and the duty cycle Fr at 30 minutes. Thecontroller uses these settings to calculate a required flow rate of 90CFM to be achieved for 10 minutes in every 30-minute period. The uppertime line 452 in FIG. 15 shows, as did the time line in FIG. 14, theperiods when the heat is and is not being called for. The lower timeline 454 in FIG. 15 shows the periods when the damper is open andclosed. In the initial 10-minute period 456, when the fan is running,the damper is opened enough to achieve a 90 CFM flow rate, as determinedby the controller. In the next, 20-minute period 458, running to the endof the half-hour, the damper is closed because the controller hasdetermined that the quota of damper open time for that half hour hasbeen met. The periods are then repeated in the second half hour. Unlikethe system shown in FIG. 14 (which does not allow the user to specifyflow rates), the desired flow rate/time schedule is met exactly in FIG.15.

Portions of the algorithm used for the main controller and the airflowcontroller are shown in FIG. 16.

At block 500, the controller accepts inputs from the user that mayinclude Af, Fp, Fm, Hold, High Occupancy, and a set point. If the userinputs have changed any of those values, 500, the system resets thecontrol algorithm accordingly 504. Otherwise the controller reads thecurrent temperature setting from the sensor in the space 506. If thecurrent temperature corresponds to the current set point, 508, thecontroller determines whether the on period of the fan has met the valueFm. If not, the controller turns off the heater or cooler (if it wasalready on) and leaves the fan on. If so, the controller turns of theheater or cooler (if it was already on) and turns of the fan and closesthe damper. Then the controller returns to check the temperature againstthe set point again.

If the temperature does not correspond to the set point, the controllerturns on the heater or cooler 516 and tests whether the on period of thefan has met Fm. If so, the controller returns to check the temperatureagainst the set point again. If not, the controller signals the airflowcontroller 518 to open the damper to position x. The airflow controlleropens the damper to position x 520 and then determines the actual flowrate using the sensor signals 522. Next the airflow controller comparesthe flow rate to Ar. If the flow rate is too low, the airflow controlleropens the damper by an increment 528; if too high, the airflowcontroller closes the damper by the increment 526. If the damper isalready fully open or fully shut, an error can be signaled by the maincontroller. If a fully open damper does not provide enough total airflow in some cases the controller could increase Fm. Or the controllercan signal an error and ask the user to check the filters. If neithertoo low nor too high, the airflow controller so indicates to the maincontroller which then again tests the temperature against the set point.

The requirement for minimum airflow in a space could be one set by anindustry standards group, for example, ASHRAE, or could be one set by auser or by a manufacturer of air handlers or by a builder of the houseor other structure. For example, the builder may know the building leaksmore than intended so that less than the recommended amount of fresh airneeds to be provided to the space. Or even tighter building techniquescould produce a need for higher than previously recommended fresh airreplacement rates Conversely it could be yet a new building method wherethe home was tighter.

By monitoring the airflow and/or the damper position over time in agiven system, it is also possible to determine when the filter needs tobe cleaned or replaced. Decreases in the airflow rate will indicateblockage of airflow. When the airflow falls below a predetermined value,an indicator can tell a user that it is time for filter maintenance. Thepredetermined value may be set empirically for systems in general, orfor each installed system in particular. Empirical analysis may not berequired, because filter maintenance time may also be inferred from theprofile of declining airflow. For example, the algorithm could watch foran abrupt change in airflow as an indicator that a filter situatedupstream of the central fan is clogged. In that circumstance, the damperwould be held open all the time and yet not be delivering the neededfresh air.

If the filter is on the downstream side of the central fan, as thefilter clogs more air will be drawn from the outside, increasing airflow and drawing in more air than is appropriate to mix with therecirculated air. In the latter case, when the filter clogs, thepressure in the air handler drops and the flow from the outside worldincreases. The algorithm would detect these events and trigger anindicator that the filter should be replaced or cleaned.

When a new filter is installed, the algorithm could determine that factautomatically by watching for a prolonged abrupt decrease or increase inair flow that lasts at least, say, 10 minutes. The algorithm could thenstore the air flow rate for the new filter. When the air flow rateincreases or decreases from the new filter rate by a change amount thatis predetermined the filter maintenance alarm would be raised.

Before a filter is fully clogged and as it becomes slowly clogged fromits new state, the algorithm will automatically accommodate the changein air flow. Thus the system will achieve both a longer effective filterlife and simultaneously achieve a more constant and precise air flowrate.

The techniques described above may be implemented in a wide variety ofmachines, including hardware, software, firmware, or combinations ofthem. The implementations may be part of or include other devices, suchas thermostats or other controllers. When microprocessors are used, theyare controlled by software that is written in or compiled into orinterpreted in their native language. The software may be stored orcommunicated in a variety of media including, for example, memory, flashmemory, mass storage devices, network based communication channels,buses, or wirelessly.

Other implementations are within the scope of the following claims.

The controller may be used not only to control dampers but also turn onand off a heat recovery ventilator (which may be used to exchange heatfrom outgoing air with the incoming air) or an in-line boost fan (whichcould be used to bring more fresh air into the system in the case oflong intake duct run, for example) or an exhaust fan (in a balancedventilation system). The airflow controller may have an auxiliary outputthat will signal anytime the damper is open (in any position). Theoutput may go to a relay board that can be used to turn on and offanything else that a user might want to control.

The air sensing unit, the damper, and the flange need not beinterconnected as an assembly and can be mounted separately or in pairs(or as the complete assembly) anywhere along the air intake duct. Theassembly can comprise any two of the three units with the third onebeing installed separately. The damper need not be custom made to coupleto the other two units, but rather can be a commercially available motordriven damper.

The airflow sensor could be implemented in a variety of ways thatinclude a rotating fan and in ways that do not involve a fan. Air flowcould be sensed using a hot wire anemometer, for example. The sensorcould be designed to measure air pressure rather than fan rotation andthe algorithm could infer air flow from changes in the air pressurewithin the intake duct.

Other algorithms could be used to determine how to control the damper toachieve a desired profile of air flow.

Controlling of the duty cycle of the damper in the fully open and fullyclosed states may be a simple and economical way to achieve a desiredaverage flow rate, and controlling of the duty cycle might be combinedwith controlling the amount of opening and closing of the damper toachieve a precise instantaneous air flow rate.

1-24. (canceled)
 25. A method comprising enabling a user of a controllerassociated with an air handler to enter a value of a minimum amount oftime that a fan of the air handler is to run.
 26. The method of claim 25also comprising enabling the user to enter a value of a duty cycle ofthe fan.
 27. A medium bearing instructions to cause a machine to enablea user of a controller associated with an air handler to enter a valueof a minimum amount of time that a fan of the air handler is to run. 28.Apparatus comprising a controller to enable a user of a controllerassociated with an air handler to enter a value of a minimum amount oftime that a fan of the air handler is to run. 29-81. (canceled)